The present invention relates to an apparatus suitable for receiving an intensity-modulated burst signal, in particular, an intensity-modulated optical burst signal.
In recent years, there has been a strong demand for the construction of Local Area Networks (LAN) and many studies and schemes have been made and carried forward for practical application. Viewed in the light of a combination of the network topology and the control procedure used, the Local Area Networks are showing the trend mainly to a loop- (or ring-) network with token passing control system and a bus-network with CSMA/CD control system. The present invention is directed to the receiving apparatus of the bus network with CSMA/CD control system. As a system using the CSMA/CD control system, there is "Ethernet" by Xerox Corp. This is disclosed, for example, in R. M. Metcalfe et al., "Ethernet: Distributed Packet Switching for Local Computer Networks", Comm. ACM, Vol. 19, No. 7 (1976), pp. 395-404. This Ethernet uses a coaxial cable as a bothway transmission line to form a bus-like transmission line. A plurality of nodes are high-impedance-coupled to the transmission line. In the case of transmitting a message from one of the nodes, it is checked first whether a packet signal from another node exists in the transmission line (CS: Carrier Sense). Where the packet signal from another node exists, i.e. where the transmission line is not idle, the packet signal transmission is deferred, and if the transmission line is idle, a message is transmitted as a packet signal over the transmission line. This signal is received by all the nodes connected to the transmission line, that is, it is arranged as an MA (Multiple Access) system, and each node inputs the packet signal addressed thereto. There is the case where a plurality of nodes simultaneously start the packet signal transmission, and in this case, a signal collision occurs. During transmission each node always monitors the signal collision and, upon detection of the collision (CD: Collision Detection), it stops the transmission, and after a random waiting time, each node starts the procedure for retransmission.
This CSMA/CD control system is utilized not only for transmission by wire but also for transmission by electromagnetic waves and optical signal. In the case of utilizing optical signal, a system employing a star coupler is widely used. That is, an optical output signal from each node is provided to the star coupler disposed as a center node. In the star coupler the optical power of the input signal from any node is uniformly distributed to each optical output terminal, and the power is again introduced into a receiving part of each node. In this case, the transmission system forms, logically, a bus topology but, physically, a star one.
It is desirable that such a burst signal receiving apparatus has short transient response time and it can reach the steady-state receiving condition, permitting reception of correct information in a short time after the start of the reception of the burst signal.
The use of a high-gain, wide-band amplifier capable of amplifying even a DC component, renders such a burst signal receiving apparatus expensive and poor in stability. For this reason, an AC-coupling amplifier is widely employed. In this case, if a DC component is contained in burst signal codes, the influence of transient response at the beginning and the terminating portion of the burst signal cannot be neglected. The transient time can be reduced by increasing the low-frequency range cutoff frequency of the AC-coupling amplifier, but in this case, a low-frequency range cutoff distortion is stressed to degrade the eye pattern, increasing decision errors. It has also been proposed to insert a DC component by using a clamp circuit, but this is defective in that a long guard time is needed when receiving a burst signal of a small level immediately after a burst signal of large level.
Further, it is necessary in the aforesaid CSMA/CD control system that the signal collision be detected without fail. Especially, in the case of the optical burst signal, there is the possibility that the level difference between two colliding signals is a maximum of about 10 dB because of deviation in the optical output, loss by the optical fiber, a loss deviation of the star coupler, connector loss and splicing loss. Even such a collision of signals of greatly different levels must be detected quickly and with certainty. As a solution to this problem, there has been proposed a system in which the mean receiving level greater than a predetermined value is decided to represent a signal collision since in the case of a plurality of burst signals arriving, the mean level of the receiving burst signals becomes higher than that in the case of only one burst signal being received, as disclosed in Japanese Patent "Kokai" (Publication) No. 159036/83 published on Sept. 28, 1983. This system is, however, defective in that variations in transmission line loss between transmitting and receiving ends, which are caused by different fiber lengths and numbers of connectors used between various transmitting and receiving ends, must be corrected by a fixed attenuator or the like, or that the decision level for detecting a signal collision has to be altered according to the variations in the transmission line loss. Moreover, there has also been proposed a system in which the mean level of optical burst signals is detected by a star coupler and when the detected level exceeds a predetermined value, a signal collision is decided to have occurred, as described in U.S. application Ser. No. 326869 filed on Dec. 3, 1981, now U.S. Pat. No. 4,531,238. In this case, an optoelectric transducer, a smoothing circuit and a level detector are provided in the star coupler to make it complex. This is not preferable since the star coupler must be of high reliability. Furthermore, there is such a system in which a transmission signal and a reception signal are exclusive-ORed at each terminal to detect a signal collision by noncoincidence between them, as described in the aforesaid literature on the Ethernet, though this system is not for the optical burst signal transmission. In this case, the transmitted signal and the received signal must be phase-adjusted so that their collision may be detected correctly.
Besides, for example, in the aforementioned "Ethernet", a signal is transmitted after being converted into a Manchester code, by which the timing component is increased and the DC component of the signal is made substantially constant without sustaining the same code long. In the past, however, the original code has been regenerated directly from the received Manchester coded signal at the receiving side.